1580s, a variant spelling of metal. Both forms of the word were used interchangeably (by Shakespeare and others) in the literal sense and in the figurative one of "stuff of which a person is made, (a person's) physical or moral constitution" (1550s), hence "natural temperament," specifically "ardent masculine temperament, spirit, courage" (1590s). The spellings diverged early 18c. and this form took the figurative sense. Related: Mettled.